In the manufacturing of web material, the web material is typically transported by means of rotatable rollers. If the web material is sensitive to scratches and abrasions, such as photosensitive web material, care must be taken that scratches and abrasions do not occur as the web is transported. Such scratches and abrasions can occur if the web material "slips" on the roller. That is, if during transport of the web material by the rollers, the web material does not move at the same speed as the rotating roller, the web material may slip across the surface of the roller resulting in scratches and abrasions. As the speed of the web material is increased during transport, slippage is more likely to occur. Indeed, depending on a particular set of manufacturing parameters (including transport speed), it may not be possible to transport a specific web material at those parameters without slippage occurring.
Generally, traction refers to an adhesive or static friction. In a web handling environment, traction typically refers to the maximum web tension differential that can be supported across a roller without slippage occurring. Various factors can affect the traction, including tension levels of the web material, the angle of wrap of the web material on the roller, and the frictional properties of the web material and roller. By knowing the traction, slippage can be avoided.
U.S. Pat. No. 4,811,591 (Antoine) relates to a device for checking the surface condition of materials. Two identical wheels are positioned across the width of the material whose surface condition is to be checked. One of the wheels is braked until slip begins. The difference in the rotational speeds of the two wheels and the braking torque is determined. The braking torque provides an indication of the relative value of adhesion and surface condition.
U.S. Pat. No. 4,909,073 (Takahashi) relates to an apparatus for measuring the coefficient of friction between a wheel and a road surface, wherein the torque on a torsion bar interconnecting two wheels is monitored as one wheel's speed is changed until slippage occurs.
While these apparatus may have achieved certain degrees of success in a particular application, the apparatus are not suitable for an on-line, web transport manufacturing process. For example, the arrangement of the two wheels across the width of the material measures the coefficient of friction at a particular widthwise location, which may not represent the aggregate coefficient of friction of the web material. Further, such an arrangement of the two wheels tends to introduce lateral forces which may steer the material. Steering the material will adversely affect the movement of the web in a manufacturing process. In addition, forces which are significant at high speeds, such as viscous drag, air entrainment, and web material bending, need to be accounted for, as well as the ability to quickly recover from an induced slip condition.
Accordingly, a need continues to exist for an apparatus to determine the traction and coefficient of friction between a web and a surface. An automatic traction measurement apparatus and method which can be incorporated on-line in a web transport manufacturing process would ensure that adequate traction levels exist on the manufacturing process at all times, thereby avoiding problems relating to loss of traction. Such an apparatus and method should be capable of measuring the overall traction across the web, and should not adversely affect the manufacturing process or the web material. The apparatus should be suited for existing web transport rollers, account for forces which are significant at high speeds, and have the ability to quickly recover from a slip condition.